The invention concerns an implantable defibrillator comprising a chargeable voltage source with a long service life and a pulse generator having a capacitor which is chargeable, preferably by way of a voltage multiplier.
For the purposes of charging up the capacitor a sufficiently large amount of energy is taken from the voltage source in order and if necessary to be able to deliver a defibrillation current pulse into the tissue to be stimulated by means of the pulse generator by way of cardiac electrodes which are connected to the capacitor terminals. The amount of energy of such a direct current surge in pulse form which is to be applied in the case of ventricular fibrillation is in the range of between 50 and 500 Ws with a pulse duration in the ms-range.
European patent application EP 0 916 365 A1 and U.S. Pat. No. 5,609,618 disclose defibrillators of that kind, wherein the voltage multiplier is in the form of a DC-DC-current transformer.
In order to achieve reliable operational management for a defibrillator, sensor means are provided in order to be able to already initiate charging of the capacitor of the pulse generator upon the occurrence of tissue conditions which on the basis of experience are to be assessed as initial symptoms of early cardiac fibrillation so that if necessary the capacitor has already received the amount of energy which is required for the direct current pulse.
In the event that cardiac fibrillation fails to occur that amount of energy has to be removed internally if the defibrillator is not activated in accordance with the prevailing safety requirements, as discussed in EP 0 916 365 A1. Energy losses which occur as a result disadvantageously lead to a premature reduction in the performance and capability of the battery so that replacement of the defibrillator by re-implantation has to take place at a correspondingly earlier moment in time.
In consideration of the deficiencies in the state of the art therefore the object of the present invention is to provide a defibrillator of the general kind set forth in the opening part of this specification, having an improved circuitry design which permits particularly energy-economical operation of the defibrillator in order to achieve an increase in the length of the change cycle of the defibrillator by virtue of an increase in the service life of the voltage source.
In accordance with the invention that object is attained by a defibrillator of the kind set forth in the opening part of this specification, with a return line between the capacitor and the voltage source which is of such a nature that electrical energy which is not consumed or which is not required for tissue stimulation can be returned by way of the return line from the capacitor to the voltage source.
The invention involves the realisation that energy can be stored in a rechargeable battery if there is applied to the battery a voltage which is of a greater value than the terminal voltage which exists at the battery at the present time. Thus it is possible for example for energy in the form of an electrical charge stored in a capacitor to be returned to a battery if the capacitor voltage exceeds the terminal voltage of the battery.
In accordance with a preferred embodiment of the invention, in the case of a defibrillator which is implanted for the stimulation of human cardiac tissue, there is provided a return line between the capacitor and the voltage source in the form of a chargeable batter, by way of which electrical energy which is not consumed or which is not required for tissue stimulation is returned from the capacitor to the battery. In that way, the capacity and performance of the battery can advantageously be improved and the service life thereof can be prolonged so that replacement of the defibrillator by re-implantation because of the battery being exhausted has to be implemented only at a later time, which is particularly desirable for the patient in question.
In accordance with an advantageous development of the invention, in the case of the defibrillator a switching device is arranged in the return line in order to be able to easily control in respect of time the return of energy to the battery from the capacitor which is charged up to the high voltage required for defibrillation. A control option of that kind is required as the operational management of the defibrillator may not be adversely affected by the return line and the return flow of energy may occur only when it is reliably established with certainty that tissue stimulation is no longer necessary.
In accordance with the preferred embodiment of the invention the switching device which is disposed in the return line is adapted to be activatable by a clock generator. Upon actuation the clock generator produces an output voltage in pulse form, wherein a time window is delimited by the pulses. The size of the time window is so selected that it is certain to involve a time region which extends at least from the moment in time at which sensor means are operable to detect a condition of the tissue in whichxe2x80x94in accordance with empirical valuesxe2x80x94tissue stimulation will be required with a very high degree of probability in a short time to the moment in time when tissue stimulationxe2x80x94if necessaryxe2x80x94has been concluded.
If the moment in time intended for tissue stimulation has passed and no tissue stimulation has been triggered off, then the switching device in the return line is activated. The connection between the capacitor and the battery is made and the amount of energy which has not been consumed is fed into the battery.
A separate control is provided for co-ordination of activation of the clock generator for controlling the switching device in the return line, and switching on the pulse generator for producing the capacitor charge which is required for a tissue stimulation procedure which is possibly to be implemented. The signal input of that control is connected to sensor means which are positioned in the tissue to be stimulated and by which the condition of the tissue is continuously monitored. The signal outputs of the control are connected to the pulse generator and to the clock generator so that, upon the occurrence of sensor signals involving corresponding signs of tissue stimulation becoming necessary in a short time, activation of the pulse generator and the clock generator is effected at the same time. The clock generator in turn activates the switching device in the energy return line, in which case the energy return line is interrupted by the first pulse which delimits the time window of the clock generator so that charging of the capacitor in the pulse generator can take place.
If, after a time which is defined by the second pulse of the time window of the clock generator, no tissue stimulation takes place, for example because the cardiac tissue has stabilised itself, then the switching device in the energy return line is activated again by the second pulse of the time window and restores the line connection between the battery and the capacitor terminal.
In accordance with an advantageous development of the invention, a time delay member is provided in the control line connecting the pulse generator to the signal output of the control, in order in a simple manner to provide that the energy return line is certain to be interrupted before charging of the capacitor to a voltage level which is required for the direct current pulse begins.
In accordance with an advantageous variant of the invention, for the time window which embraces at least a region from the moment in time at which the pulse generator is switched on to the theoretical moment in time at which a required tissue stimulation procedure is triggered off, there is provided a safety time range which is of a value of at least 10% of the minimum time window required.
Another advantageous variant of the invention provides for using a telemetrically programmable clock generator in order conveniently to be able to implement patient-specific adjustment of the time window.
Other advantageous developments of the invention are characterised in the appendant claims or are set forth in greater detail hereinafter together with the description of the preferred embodiment of the invention with reference to the drawing in which: